In order to address the issue of increasing bandwidth requirements demanded for wireless communication systems, different schemes are being developed to allow multiple user terminals to communicate with a single base station by sharing the same channel (same time and frequency resources) while achieving high data throughputs. Spatial Division Multiple Access (SDMA) represents one such approach that has recently emerged as a popular technique for the next generation communication systems.
In SDMA systems, a base station may transmit or receive different signals to or from a plurality of mobile user terminals at the same time and using the same frequency. In order to achieve reliable data communication, user terminals may need to be located in sufficiently different directions. Independent signals may be simultaneously transmitted from each of multiple space-separated antennas at the base station. Consequently, the combined transmissions may be directional, i.e., the signal that is dedicated for each user terminal may be relatively strong in the direction of that particular user terminal and sufficiently weak in directions of other user terminals. Similarly, the base station may simultaneously receive on the same frequency the combined signals from multiple user terminals through each of multiple antennas separated in space, and the combined received signals from the multiple antennas may be split into independent signals transmitted from each user terminal by applying the appropriate signal processing technique.
A multiple-input multiple-output (MIMO) wireless system employs a number (NT) of transmit antennas and a number (NR) of receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS spatial streams, where, for all practical purposes, NS min{NT,NR}. The NS spatial streams may be used to transmit NS independent data streams to achieve greater overall throughput.
In a multiple-access MIMO system based on SDMA, an access point can communicate with one or more user terminals at any given moment. If the access point communicates with a single user terminal, then the NT transmit antennas are associated with one transmitting entity (either the access point or the user terminal), and the NR receive antennas are associated with one receiving entity (either the user terminal or the access point). The access point can also communicate with multiple user terminals simultaneously via SDMA. For SDMA, the access point utilizes multiple antennas for data transmission and reception, and each of the user terminals typically utilizes less than the number of access point antennas for data transmission and reception. When SDMA is transmitted from an access point, NS=min{NT, sum(NR)}, where sum(NR) represents the summation of all user terminal receive antennas. When SDMA is transmitted to an access point, NS=min{sum(NT), NR}, where sum(NT) represents the summation of all user terminal transmit antennas.